Electrophotographic image forming apparatuses are of two types: direct transfer types and indirect transfer types. In direct transfer types, an electrostatic latent image formed on an image bearing member (either drum-shaped or endless-belt-shaped), such as a photoreceptor, is developed into a toner image with the charged toner particles, and the toner image is then directly transferred from the image bearing member onto a sheet-like material, such as copy paper, resin sheets, thick paper, and postcards, on which toner images are fixable. In indirect transfer types, the toner image is transferred from the image bearing member onto an intermediate transfer member (either drum-shaped or endless-belt-shaped) first, and then transferred onto the sheet-like material. The sheet-like materials are hereinafter collectively referred to as “paper” for convenience. Typically, but not necessarily, paper is the medium from which is made a sheet on which an image is to be formed. Other printable media is available in sheets and their use here is included.
In both direct and indirect transfer types, some toner particles remain on the image bearing member without being transferred onto paper. Such residual toner particles are typically scraped off with a blade-like cleaning member or electrically removed with a charging member (e.g., a charging roller) by applying an electric field having the opposite polarity to the toner particles. This process is hereinafter referred to as “cleaning process”.
The image bearing member, cleaning member, and charging member are subjected to physical and electrical stresses in the cleaning process, resulting in abrasion and deterioration of these members. To solve this problem, various attempts have been made.
For example, Examined Japanese Patent Application Publication No. 51-22380 proposes forming a lubricative film with zinc stearate on the outermost surface of an image bearing member so as to protect the image bearing member from abrasion with a cleaning member.
Unexamined Japanese Patent Application Publication No. 2001-305907 also proposes forming a lubricative film on the outermost surface of an image bearing member. The lubricative film is formed in a way in which a brush roller scrapes a solid lubricant and supplies the scraped powdered lubricant to the outermost surface of an image bearing member. The supplied lubricant is evened out with a blade so that the resulting lubricative film has a uniform thickness which prevents the production of defective images.
Because the solid lubricant is pressed against the brush roller with a spring, there is a problem that the pressing force of the spring gradually weakens and the amount of the protective agent supplied gradually reduces as the solid lubricant is consumed.
Unexamined Japanese Patent Application Publication No. 2007-293240 attempts to solve this problem and provides a lubricant applicator that is able to supply a constant amount of lubricant to an image bearing member regardless of the degree of consumption of the solid lubricant. Also, Unexamined Japanese Patent Application Publication Nos. 2001-305907 and 2007-293240 both describe that zinc stearate is preferable for the lubricant.
The image bearing member is also subjected to electrical stresses when being charged (hereinafter “charging process”), notably by contact charging or proximity charging that causes electrical discharge. Contact or proximity charging has a problem that various active species and reactive products, to be adsorbed to the surface of the image bearing member, generate due to electrical discharge.
In the charging process, zinc stearate behaves as a suitable lubricant for protecting the surface of the image bearing member from electrical stress. However, in the cleaning process, zinc stearate undesirably allows toner particles to slip through the blade-like cleaning member due to its lubricity. This results in deterioration of not only the cleaning member but also the resulting image quality. Moreover, toner particles having slipped through the cleaning blade contaminate the charging member, resulting in production of defective images. Sipping though the blade-like cleaning member is more likely to occur when the toner shape is more spherical.
There is another problem that zinc stearate is likely to deteriorate by electrical stress. Deteriorated zinc stearate has no more lubricity and accelerates slipping through of toner particles and contamination of the charging member. In view of this, Unexamined Japanese Patent Application Publication No. 2006-350240 proposes a mixture lubricant comprised of zinc stearate (i.e., a metal salt of a fatty acid) with boron nitride (i.e., an inorganic lubricant) that does not degrade its lubricity even under electrical stress.
One typical method of forming powdered lubricant into solid block includes melt molding. When a mixture lubricant including about the same amounts of zinc stearate and boron nitride is subjected to melt molding, the resulting solid block is too hard to be scraped off. This problem can be solved by forming solid block by compression molding. Alternatively, reducing the content ratio of boron nitride in the mixture lubricant is effective for reducing the hardness of the resulting solid block, while the content ratio of boron nitride correlates with the degree of contamination of the charging member. The smaller the content ratio of boron nitride, the worse the degree of contamination of the charging member.
Unexamined Japanese Patent Application Publication Nos. 2009-150986 and 2009-169237 each disclose a lubricant applicator in which a solid lubricant formed by melt-molding zinc stearate is scraped off by a closed-cell foam roller. The scraped solid lubricant is formed into particles having a relatively small and uniform particle size, which can form a uniform protective layer on an image bearing member.
However, this lubricant applicator has not solved the problem of deterioration of zinc stearate under electrical stress that accelerates slipping through of toner particles and contamination of the charging member. In addition, when the closed-cell foam roller keeps contacting both the image bearing member and the solid lubricant for an extended period of time, it is likely that the closed-cell foam roller causes distortion because closed-cell foams generally cause greater compressive residual distortion than open-cell foams.